This application claims priority to an application entitled xe2x80x9cHandoff Device and Method For Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on May 8, 1998 and assigned Serial No. 98-16454, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to mobile communications systems, and in particular, to a system and method for determining a handoff target base station in a mobile communication system.
2. Description of the Related Art
A code division multiple access (CDMA) mobile communication system performs handoff to continuously provide a call service, even when a mobile station travels from a present cell to an adjacent cell during the call service. There are two types of handoff: a soft handoff and a hard handoff. In the soft handoff, the mobile station maintains a call using both a channel assigned by a handoff target base station to which the mobile station is to be handed off and a channel assigned by the present base station in service. Meanwhile, one of the two channels, whose channel condition has a value lower than a threshold, is released. In the hard handoff, a channel assigned by the present base station in service is first released and then, a connection to an adjacent base station is attempted.
In general, a mobile station, during a call, monitors the strength of pilot signals transmitted by base stations adjacent to or in proximity to the mobile station. The mobile station stores the pilot signals having a relatively higher strength in a table. The base stations included in the table are referred to as xe2x80x9cadjacent (or handoff candidate) base stationsxe2x80x9d. As the mobile station travels, the pilot signals from the adjacent base stations vary in strength, and the mobile station changes the values stored in the table. The mobile station then provides information about the pilot signal strengths to the base station in service. Then, upon receipt of the pilot signal strength information, the base station in communication with the mobile station determines whether to perform a handoff operation. A base station having a pilot signal which has the highest strength, becomes a handoff target base station to which the mobile station is to be handed off. However, determining the handoff by simply measuring the strength of the received signal increases the frequency of the handoff operation resulting in an increase of a system load.
With reference to FIG. 1, there is shown a diagram illustrating a mobile communication system. A mobile communication network includes mobile switching center (MSC) 141 connected to multiple base station controllers (BSCs) 131-13n, each of which is connected to multiple base stations 121-12n. Mobile stations 111-11n belonging to one cell are wirelessly connected to corresponding base station 121 to provide a call service. Multiple base stations 121-12n are controlled by the BSC 131. The BSCs 131-13n are then controlled by the MSC 141. If the mobile station 111 attempts to communicate with a wire subscriber 161 belonging to a public switched telephone network (PSTN) 151, rather than with another mobile station belonging to the mobile communication network, the MSC 141 establishes a traffic channel by connecting the wire subscriber 161 to the mobile station 111 via the PSTN 151. Alternatively, it is also possible to unify the base stations 121-12n and the corresponding BSC 131 into one body.
A description will now be made to a conventional handoff operation performed by the mobile communication system of FIG. 1 with reference to FIG. 2. When a mobile station MS5, which is being presently serviced by a base station BS1, travels to a cell area CELL2 of an adjacent base station BS2, the handoff occurs at an overlapped area OL of a service area (or coverage) CELL1 of the base station BS1 and a service area CELL2 of the base station BS2 under the control of a BSC (not shown). That is, as the mobile station MS5 being serviced by the base station BS1 moves to the overlapped area OL, it measures a strength, T_add, of a signal received from the adjacent base station BS2 and provides the measured strength information to the BSC. The BSC then determines whether the base station BS2 can service the mobile station MS5, and provides the service to the mobile station MS5 via both the base station BS1 and the base station BS2 when the base station BS2 can provide the service.
If the strength of a signal received from the base station BS1 decreases below a threshold T_drop as the mobile station MS5 travels to the base station BS2, a radio link connected to the base station BS1 presently in service is released and the service is provided via a radio link connected to the base station BS2. Here, the release of the radio link does not occur as soon as the signal strength of the base station BS1 drops below the threshold T_drop, but the base station BS1 is dropped when the signal strength is continuously maintained below the threshold T_drop for a guard time Txe2x80x94Tdrop. 
In some cases, the mobile station MS5 may select a remote base station as the handoff target base station, instead of an adjacent base station or a base station which is the closest in proximity to the mobile station MS5. For example, referring to FIG. 3, when the mobile station MS5 travels from the base station BS1 to the base station BS2, the mobile station MS5 determines a handoff target base station, depending on strengths of pilot signals transmitted from the base stations. Here, for handoff, it is preferable for the mobile station MS5 to select a base station which is most adjacent thereto.
However, the signal strength of the nearest base station BS2 may be lower than a signal strength of a remote base station BS3 due to an obstacle such as a building. In this case, the mobile station MS5 may be handed off to the remote base station BS3 rather than the nearest base station BS2, depending on the signal strength. However, as the mobile station MS5 continues to travel, the channel condition is changed such that the signal strength of the presently connected base station BS3 decreases and becomes lower than the signal strength of the nearest base station BS2. Then, the mobile station MS5 performs a handoff again. In a worst case scenario, an abrupt drop in the signal strength of BS3 may cause the call service to be terminated, thereby disconnecting the call.
That is, when the signal strength of the base station BS3 is higher than T_add in an instant due to the physical surroundings of the mobile station MS5, the mobile station MS5 performs a handoff add process for adding the base station BS3 to an active set. However, in the next instant, when the signal strength of the base station BS3 decreases, the mobile station MS5 performs a handoff drop process to drop the base station BS3 from the active set. In this case, the control signals exchanged for the handoff may increase system load, and the base station BS3 added to the active set may unnecessarily assign a traffic channel to the mobile station MS5, thereby reducing channel resources.
It is, therefore, an object of the present invention to provide a handoff system and method for performing: handoff depending on strengths of signals received from base stations in proximity to a mobile station, and weights based on the relative location of the mobile station with respect to each of the base stations in proximity thereto.
It is another object of the present invention to provide a handoff system and method for performing handoff depending on the direction of travel of the mobile station.
It is still another object of the present invention to provide a handoff system and method for performing handoff depending on a traveling velocity of the mobile station.
To achieve these and other objects, a system and method for determining a handoff target base station in a mobile communication system according to a direction of travel of a mobile station are provided. The mobile station measures strengths of pilot signals transmitted from base stations in proximity thereto and transmits information about the measured strengths to a base station controller (BSC). The BSC selects one of the base stations as the handoff target base station depending on the measured strength information and relative location of the mobile station with respect to each of the base stations. Weights corresponding to each of the base stations are also taken into consideration to select the handoff target base station. The weights are determined by determining a direction of travel of the mobile station relative to each of the base stations, where the direction of travel is based upon the direction of movement of the mobile station.